Electric wave filter



Jan. 11,1927. 3, 52

K. S. JOHNSON ELECTRIC WAVE FILTER Filed Dec. 31 1920 2 Sheets-Sheet 1 [TZTZ ,..Wi 7?]. h

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Patented Jan. 11,1921.

UNITED STATES PATENT-OFFICE.

xENNETx s. JOHNSON, F JERSEY CITY, NEW JERSEY, ASSIGNOR To WESTERN ELEc-' TBIC COMPANY, INCORPORATED, o NEw YORK, N. Y., A CORPORATION or NEW YORK.

ELECTRIC- WAVE FILTER.

Application filed December 31,1920. Serial 170.4%,388.

This invention relates to electric wave filters adapted to be employed in a line such as a telephone line, to prevent the passage of currents of certain frequencies and permlt the passage of currents having other frequencies.

It has for its object to produce filters of various ty )es which have the characteristics of types 0 filters in general already known.

but which may be manufactured from a less amount of material, thus reducing the cost and size of the filter.

In accordance with the invention, filter sections are employed which have similar 5 impedances spaced apart in the direction of current propagation but having mutual impedance therebetween, and which have one or more impedanccs located between the mutually related impedances. The effect of mu- 0 tual impedance in general is to add an impedance to the common portion without the use of additional apparatus.

The invention will be more fully understood from the following detailed descrip- 'tion and claims, taken in connection with the accompanying drawings in which Fig. 1 rep resents schematically, a filter section em: bodying the invention, Figs. 2, 3 and 4 represent modified forms of the invention.

.30 Fig. 5 represents a filter having a plurality of sections of the type shown in Fig. 2; Fig. 6 represents a filter without mutual inductance which is electrically the equivalent of Fig. 5. and Figs. 7 and 8 represent filter sections which are the equivalents of Figs. 1 and 4 respectively.

In Fig. l is shown a filter section having a shuntcondenser C and series of impedances each comprising a condenser C and an in- 4 ductance L in parallel. The inductances L, are wound in series opposing relation on the core 8. By series opposing relation is meant that relation in which the coils are connected in series in such a direction that the fiux produced by the current in one coil is opposed or decreased by that produced by the current flowing in the other coil. This results in a transformer action and as is well known, is equivalent to the insertion of an additional self inductance in a branch comductanceis shown at M, in Fig. 7. -The inductance due to magneticleakage remains as shown at L,M in Fig. 7.

Fig. 1 may be regarded as a general case of which Fig. 2 is a specific form having only inductance in its series arms. It L denotes the self-impedance of each winding, M the mutual impedance, and C the impedance of the shunt condenser in a section of the filter shown in Figs. 2 and 5, and if the impedances in Fig. 6 have the values indicated on the drawing, it can be shown by Kirchhofis laws that a section of the former is equivalent to a section of the latter. Assume that equal electromotive forces E are applied to the left hand side of the first sectionin Figs. 5 and 6, and that the first sections are short-circuited at mid'series at the right. A mid-series termination is one which includes one half of the series inductance of the terminal section. Designating the total current entering each filter as I and I respectively, and the currents in the condensers by I and I respectively, then by Kirchhoffs second law, the currents in the short-circuited branch are I I and I 'I respectively. By Kirchhofis first law we obtain from Fig. 5 by going around the first mesh:

I :I L-M)+ C( and by going around the outer mesh:

EIILL L-1 M+ I I L-JLM, or

E:I (2L2M)I (LM) (2) In Fig. 6 by going around the first mesh we obtain:

J J and by going around the outer mesh:

12:1. (L"M) (I 'I 1PM) By comparison of equations (1) and (2) I Figs. 5 and 6 are equal.

Z2 =0 (5) and Z 1 2 4 (6) In the filter of Fig. 6, Z =j2(L-.M) m, and

in which ;i= 1, L and M represent inductance, C represents capacity, and e::21rf, in which represents frequency. Combining (7) and (8),

Z, 2(LM)w 2(LM)Cw Z l MCaF- 1 Assuming 0, we get from equation 9,

w=0 orlf=O Assuming g-Z -4, we get f: ac L M (10) Therefore, the filter of Fig. 6, and hence that of Fig. 5, also, will transmit freely all frequencies between 0 and It was shown in Patent No. 1,227,113 above referred to that the propagation constant I of a filter of this type is determined by the equation:

l P-cosh 1 2 -l-1) (11) This may be expressed:

Since the quantity under the radical will be infinite when MG... =1, it is evident from equation (13), that the attenuation will be a maximum when MCw II, or

21r /l\l (l This is the condition of resonance in the shunt arm. If the frequency of resonance as determined by equation (14) be denoted by f the cut-off frequency, as determined by e nation (10), by f and the ratio 1:! b a,

it will be seen that any desired sharpness of cut-oif can be obtained by assigning a positr'e value to a more or less approaching unity.

If the core 8 be specially designed to be free from hysteresis and eddy currents there will be very little resistance effectively in the shunt arm of Fig. 2 as compared with that of a shunt arm of Fig. 6, since the latter contains the direct current resistance of the winding of inductance M. It follows that the maximum attenuation of a filter of the type shown in Figs. 2 and 5 can be made to approach more nearly infinity than that of Fig. 6.

In Fig. 3, is shown a convenient arrangement for varying the self-impedances and the mutual impedance of the windings in a filter section of the type shown in Fig. 2. The coils L are wound on sleeves 10 and 11. Sleeve 10 may be mounted on a support (not shown) and sleeve 11 is pivotally connected to sleeve 10 as indicated at 12. Cores 14 and 15 are mounted for endwise movement in the sleeves 10 and 11. It is apparent that the magnetic leakage, and consequently the mutual inductance, may be varied by rotating the sleeve 11 about its pivot 12, and that the self-inductances may be varied by sliding the cores'14 and 15 in and out.

By methods similar to those used above in connection with Figs. 5 and 6, it can be shown that the sections shown in Figs. 1 and 4 are electrically equivalent to sections having the forms shown in Figs. 7 and 8 respectively. The characteristics of a filter having sections of the form of Figs. 4 or 8 are similar to those of the filter shown in Fig. 8 of the Campbell Patent No. 1,493,600, above referred to. That is, it is a high ass filter which may be given a sharp cut-o by making the frequency of resonance approach the frequency of cut-off.

What is claimed is:

1. An electrical network comprising a condenser, means for connecting said network between two sections of a circuit and an inductance between said. condenser and each of said connections said inductances being so related as to have mutual inductance and being connected in series opposing rela tion.

2. An electrical network adapted to be connected between two sections of a circuit, said network comprising a shunt condenser, and an inductance at each side of said condenser, said inductances being so related as to have mutual inductance and being connccted in series opposing relation.

3. An electrical network adapted to be connected between twosections of a circuit, said network comprising an impedance element, and an inductance at each side of said impedance element, said inductances being so related as to have mutual inductance so as to effectively add positive inductive reactance to the impedance element.

4. In a filter section having symmetrical halves connected in series with respect to current propagation and an inductance in each of said halves, said inductances being so related as to have mutual inductance and being connected in series opposing relation with respect to said current propagation, and an impedance element common to said halves.

5. In a filter section having symmetrical halves arranged in series with respect to current propagation, a series inductance'in each of said halves, and shunt capacity common to said halves, said inductances being so related as to have mutual inductance which resonates with the shunt capacity at a frequency in the attenuated range.

6. A wave filter section having two pair of terminals for connection to a line or other filter sections, a shunt path comprising a condenser between said pairs of terminals, an inductance at each side of said shunt path having mutual inductance with the inductance at the opposite side, and being connected in series opposing relation therewith.

7. A filter section having two pairs of terminals for connection to a line or other filter sections, a shunt path comprising a condenser between said airs of terminals, inductances at either side of said shunt path, an inductance at one side of said shunt path being wound on a common magnetic struc ture in series opposing relation with an inductance on the opposite side.

8. A wave filter comprising series inductances having mutual inductance thcrebetween, and a shunt capacity havin a terminal connected between said in uctances, whereby mutual inductance is etfectivel in series with said capacity, said elements lav ing such relation to each other that the ratio of the frequency of resonance of the shunt path to the cut-ofi' frequency of the filter closely approaches unit 9. A wave filter comprising a plurality of reactive elements and a path for shunting out frequencies to be suppressed, one ofsaid "reactiveelemenls having mutual reactance with an element electrically more distant from the source of currents, said reactive elements being so proportioned that the ratio of the frequency at which maximum sup- .pression occurs to the frequency at which suppression begins closely approaches unity.

10. A wave filter comprising two inductive windings, a core .for each of said windings, said cores being pivotally connected together.

11. A wave filter comprising two inductive windings, a core for each of said wind- I ings, said cores being mounted for relative movement to vary the mutual inductance of said windings.

12., A wave filter comprising two inductive windings, a core for each of said windings, said cores being-mounted for relative movement to vary the mutualinductance of said windings, and for reciprocal movement in their respective windings.

13. A Wave filter comprising a condenser and at least two inductive windings having a common terminal, and variable means for producing a series opposing coupling between said windings.

14. In a wave filter section having impedances in series connection and in shunt connection with respect to the line, a reactive impedance disposed in the one type of connection, and a circuit branch includin an inductance disposed on each side of said impedance in the other type of connection, said inductances bein inductively coupled in series opposin re ation between terminals of said circuit ranches on one side of the line, and the impedances of said circuit branches being proportioned to. cooperate with the said reactive impedance to permit the free passage of a broad band of wave frequencies through the filter section, while at the same time attenuating waves of all other frequencies.

In witness whereof I hereunto subscribe my name this 29th day of December A. D., 

